1. Field of the Invention
The present invention relates to a system for separating particulate matter carried in a gas composition, and more particularly to a system for cleaning up gasses emitted by a fuel burning unit prior to releasing said gases into the environment.
2. Description of the Prior Art
The problems associated with removing particulate matter from gas compositions, and particularly the problems associated with removing particulate matter from the stack gases of a coal burning furnace are well known to those skilled in the art. In addition to emitting particulate matter, coal burning furnaces emit a significant amount of sulphur dioxide and alkali metal compounds, the removal of which presents a problem of a different dimension than the removal of particulate matter.
In the past, filters and cyclone separators of various designs have been used for the removal of solid particles from carrier gas compositions such as stack gases, and gaseous products of coal gasification plants. Because filters and cyclone separators in high temperature applications are incapable of removing particles of less than 10 micron in size, electrostatic precipitators have usually been employed for the removal of such smaller particles from the stack gases and the like.
Electrostatic precipitators, however, suffer from the disadvantages of high costs of initial construction and of relatively high energy input requirements. However neither filters, cyclone separators, nor electrostatic precipitators remove sulphur dioxide gas from stack gases. Consequently absorption towers and like systems have been resorted to in the past to reduce the sulphur dioxide content of stack gases through absorption in water to a more or less acceptable level. Overall, the inability of the prior art to provide for effective clean-up of stack gases at an acceptable cost has been a major factor in preventing large scale utilization of coal as a major source of electric power in the United States.
In a report submitted to the United States Department of Energy, Division of Coal Conversion and Utilization, under contract EF-77-C-01,2709, written by Hans D. Lindhardt, titled "Investigation of High Velocity Wedge Separator for Particle Removal in Coal Gasification Plants" an experimental system was described for the removal of particulate matter from a carrier gas composition produced in a coal gasification plant.
The system described in the above referenced report utilizes a wedge or a substantially wedge like obstruction positioned in a supersonic flow of the carrier gas to alter the flow path of the carrier gas while leaving the flow path of the particulate matter unaffected. A receiving member defining an appropriately positioned slot is then placed into the flow path of the particulate matter to capture the same. The report suggests that an array of separator elements or units, each unit including a nozzle wherein the carrier gas is accelerated to a supersonic speed, a substantially wedge shaped obstruction and a receiving member could be used in a system scaled for industrial use.
The above referenced report contains an extensive mathematical treatment of the relationship of the several physical parameters which must be considered in the design of the supersonic particulate matter separator described in the report. Briefly stated, these physical parameters include the density of the carrier gas, the density and size of the particulate matter, the velocity of the carrier gas flow and the dimensions of the elements or units. The density of the carrier gas and the density and size of the particulate matter is determined by the gasification process for which the separator system is designed. On the other hand, the velocity of the carrier gas flow and the dimensions of the elements or units of the separator system represent design features which must be optimized for the effective functioning of the system.
As it is described in detail in the above referenced report, the substantially wedge shaped obstruction placed into the path of the supersonic flow of the carrier gas creates a shock zone which is disposed obliquely to the wedge shaped obstruction. The term shock zone refers to a substantially stationary zone in the gas flow wherein a sudden discontinuity of the direction of the gas flow occurs. Although downstream of the oblique shock zone the flow of the carrier gas becomes parallel with the walls of the wedge shaped obstruction, the flow path of the particulate matter is substantially unaffected by the shock zone. This phenomenon provides the theoretical basis for separation of the particulate matter from the carrier gas.
The above mentioned extensive mathematical treatment of the behavior of particulate matter under the conditions of supersonic flow downstream of the shock zone reveals that the momentum range (.LAMBDA.) which itself incorporates densities, particle size and drag coefficient constants, represents a basic scaling parameter. The momentum range (.LAMBDA.) has been well defined by two phase flow fluid mechanics theory. The drag coefficient is a standard measure of bodies moving in gas streams, as is explained in standard treatises of physics and fluid flow mechanics.
The relationship between the momentum range (.LAMBDA.) and the drag coefficient (C.sub.D) is given by equation 1, EQU .LAMBDA..perspectiveto.8/3 .rho..sub.s /.rho..sigma./C.sub.D equation 1)
wherein .rho..sub.s and .rho. respectively represent the density of the particulate matter and of the carrier gas, and .sigma. represents the radius of the particles of the particulate matter. The drag coefficient (C.sub.D) is related to the Reynold's number in such a manner that in a high velocity turbulent flow the drag coefficient value is small, in the neighborhood of unity.
A basic relationship between the momentum range (.LAMBDA.), the velocity of the gas (Vo) and particle flow (Vpo) and the wedge angle (.alpha.) was derived and resulted in equation 2. ##EQU1## In the above equation .LAMBDA.x represents the movement of the particle in a direction perpendicular to the wedge and .DELTA.y represents the movement of the particle parallel to the wedge. Equation 2 represents a good approximation of dimensionless particle trajectories downstream of the oblique shock zone. It can be seen from equation 2 that the particle trajectories are strongly dependent on the momentum range (.LAMBDA.) which therefore strongly correlates the overall dimensions of an element of the separator system. It is also apparent from equation 2 that particle trajectories are much less sensitive to the velocity of the gas flow as long as the gas flow is substantially in the 0.2-6.0 Mach range.
It was on the basis of equation 2 that actual particle trajectories were calculated, plotted, and the experimental separator system described in the above referenced report was designed.
Although the above described separator system appears capable of providing a viable alternative to the electrostatic precipitators to remove particulate matter from a gas carrier, it suffers from certain disadvantages. It is not capable of effectively removing particles of radius smaller than 1 micron. Moreover, the dimensions of each element or unit of the separator system which includes a nozzle, a substantially wedge shaped member, and a receiving member are of such small magnitude that construction of the system for industrial scale application appears to be expensive. As an example it is noted that the width of a duct provided in each element of the separator system between the wedge shaped member and a wall of the nozzle is of the 0.025 inch magnitude.
Finally, the separator system described in the above referenced report does not contemplate its adaptation for the clean up of stack gases of a coal burning furnace with the attendant problem of removing water vapor and sulphur dioxide together with the removal of particulate matter. In addition this separator system appears to cause an unacceptably high pressure loss in the overall gas flow.
In light of the above, it is readily apparent that there is a need in the art for an improved separator system such as the separator system of the present invention.